In recent years, with the advent of advanced information society, the demand of a personal computer, a car navigation system, a portable terminal unit, a telecommunications system or these combined products is increasing. A thin, lightweight, and low power consumption display device is suitable for a display means of these products and a liquid crystal display module or a display module that uses an electrooptic element, such as a self light emission type EL element or an LED is used.
The display module that uses the self light emission type electrooptic element of the latter is provided with features, such as good visibility, a wide viewing angle, and suitability for a motion image display with a fast response, and is assumed to be suitable for an image display in particular.
A display that uses an organic EL element (also called an organic light emitting diode, and may also be hereinafter abbreviated to an OLED) of which the emitting layer has organic matter in recent years is greatly expected as an OLED display in cooperation with a rapid improvement of luminous efficiency and the progress of network technology that enables visual communication. The OLED display has the diode structure in which an organic light emitting layer is sandwiched between two electrodes.
In order to increase the power efficiency in the OLED display constituted using such OLED, as described later, an active matrix driving method in which a thin film transistor (hereinafter referred to as a TFT) is used as a switching element of a pixel is effective.
An art that drives an OLED display in the active matrix structure is described in Japanese Patent Application Laid-open No. HEI04-328791, Japanese Patent Application Laid-open No. HEI08-241048, or the U.S. Pat. No. 5,550,066, for example, and an art related to a driving voltage is disclosed in International Publication No. WO98/36407.
A typical pixel structure of the OLED display has a pixel driving circuit (also hereinafter referred to as a pixel circuit) including two TFTs (the first TFT is a switching transistor and the second TFT is a driver transistor) that are first and second active elements and a storage capacitance (data signal holding element, that is, a capacitor), and this pixel circuit controls the emitting luminance of an OLED. A pixel is arranged in each intersection unit in which M data lines to which a data line (or an image signal) is supplied and N scanning lines (also hereinafter referred to as gate lines) to which a scanning signal is supplied are arranged in a matrix of N rows multiplied by M columns.
For the drive of a pixel, a scanning signal (gate signal) is sequentially supplied to N rows of gate lines and a switching transistor is set to the on state (turned on). Subsequently, the scanning in the vertical direction is finished once within a one-frame period Tf and a turn-on voltage is re-supplied to the first (first-row) gate line.
In this driving scheme, the time when the turn-on voltage is supplied to a gate line is less than Tf/N. Usually, about one sixtieth second is used as the value of the one-frame period Tf. While the turn-on voltage is being supplied to a certain gate line, all switching transistors connected to the data line are set to the on state, and a data voltage (image voltage) is supplied to M columns of data lines simultaneously or sequentially synchronizing with the on state. This is usually used by an active matrix liquid-crystal display.
A data voltage is stored (held) in a storage capacitance (capacitor) while a turn-on voltage (hereinafter, turn-on is also merely referred to as ON. Equally, turn-off is also merely referred to as OFF) is supplied to a gate line, and is kept in almost their value for a one-frame period (or one-field period). The voltage value of the storage capacitance specifies the gate voltage of a driver transistor.
Accordingly, the value of the current that flows into the driver transistor is controlled and light emission of an OLED is controlled. The response time until voltage is applied to the OLED and the light emission starts is usually less than 1 μs, and even an image (motion image) of a quick movement can be followed up.
Incidentally, in an active matrix driving method, because light emission is performed over a one-frame period, high efficiency is realized. The difference is clear in comparison with a passive matrix driving method in which diode electrodes of an OLED are directly coupled to a scanning line and a data line respectively and driven without providing any TFT.
In the passive matrix driving method, because the current flows into the OLED only while the scanning line is being selected. Accordingly, to obtain the same luminance as the light emission of a one-frame period from only the light emission of the short period, the emitting luminance multiplied by almost the number of lines is required in comparison with the active matrix driving. To attain the purpose, the driving voltage and the driving current must inevitably be increased. However, a power consumption loss, such as generation of heat, is increased and the power efficiency is decreased.
Thus, the active matrix driving method is assumed to be more superior to the passive matrix driving method from the standpoint of a reduction in power consumption.